Deep boreholes, e.g., wells for fossil fuel recovery, are conventionally drilled in sections of progressively smaller diameter. As each section is drilled a tubular casing is cemented in place to line and stabilize the borehole. The next section of the borehole must then be drilled in a smaller diameter so that the drill bit is able to pass through the installed casing.
When an MWD tool is used it must be of sufficiently small diameter as to allow it to pass through the last installed, i.e., smallest diameter, section of casing in the borehole and into the section of the borehole being drilled.
However, it becomes more difficult to provide an MWD tool having the required performance characteristics as the maximum allowable diameter decreases. Furthermore, the functional efficiency of an MWD tool may be reduced under the very severe conditions encountered in the lower portion of a deep borehole.
What is needed in the art is an effective reliable MWD survey or logging tool for use in small diameter deep boreholes.